1.1 Field of the Invention
The present invention relates generally to the field of molecular biology. More particularly, certain embodiments concern methods and compositions comprising DNA segments, and proteins derived from bacterial species. More particularly, the invention provides gene compositions encoding a decorin (Dcn) binding protein (DBP) from Borrelia burgdorferi and the corresponding peptide epitopes and protein sequences comprising native and synthetically-modified Dcn binding site domains. Various methods for making and using these DNA segments, DNA segments encoding synthetically-modified ligand binding site domains, and native and synthetic proteins are disclosed, such as, for example, the use of DNA segments as diagnostic probes and templates for protein production, and the use of proteins, fusion protein carriers and peptides in various pharmacological and immunological applications.
1.2 Description of the Related Art
1.2.1 Lyme Disease
Lyme disease (Steere, 1989), or Lyme borreliosis, is transmitted by ticks, particularly of the genus Ixodes, and caused by spirochetes of the genus Borrelia. Lyme disease agents, that is borrelias isolated from humans or animals with clinical Lyme disease, are currently classified into at least three phylogenetic groups: B. burgdorferi sensu stricto, B. garinii, and B. afzelii. Strains potentially representing other phylogenetic groups of Lyme disease agents as well, such as group 25015, have been also isolated from ixodid ticks. Collectively these spirochetes are referred to as B. burgdorferi sensu lato, or simply B. burgdorferi. The genotypic and phenotypic variation among Lyme disease agents supporting the designation of these phylogenetic subgroupings is a major complicating factor for the design of effective vaccines or immunotherapeutic strategies for Lyme disease.
Lyme disease is transmitted through the bite of a tick which attaches itself to the host and, upon feeding, deposits the spirochetes into the dermis of the skin. In the skin, B. burgdorferi replicates before endovascular dissemination to organs. Typically, an annular spreading skin lesion, erythema migrans, forms from the site of the tick bite. Early symptoms of Lyme disease are flu-like and may include fatigue and lethargy. Left untreated, Lyme disease can develop into a chronic, multisystemic disorder involving the skin, joints, heart, and central nervous system.
Once deposited in the dermis, the spirochetes become associated with and appear to colonize the collagen fibers. Skin is the most consistent site of spirochete-positive culture. In persistent infection, the skin may provide a protective niche for replication, thereby acting as a reservoir of spirochetes for subsequent distribution to other tissues.
As B. burgdorferi disseminates to other organs, the organisms appear to localize to the extracellular spaces of these tissues as well. In several organs, including tendon (Barthold et al., 1993; 1991), ligament (Haupl et al., 1993), heart (Zimmer et al., 1990), and muscle (Barthold et al., 1992; Duray, 1992), B. burgdorferi spirochetes are found primarily in close association with collagen fibers, suggesting that this association is an important mechanism of tissue adherence in different stages of infection. Although the association of B. burgdorferi with collagen fibers has been reported previously by several investigators, the molecular mechanism responsible is not known. Lyme disease is typically treated with antibiotics, which are generally effective in the early stages of the disease. Later stages involving cardiac, arthritic, and nervous system disorders are often non-responsive.
1.2.2 Existing Vaccines for Prevention of Lyme Disease
Several proteins present on the outer surface of B. burgdorferi have been identified, including OspA (31 kDa), OspB (34 kDa), OspC (22 kDa), OspD, OspE, and OspF. Laboratory studies have shown that passively-administered antibodies (Schaible et al., 1990) reactive with the B. burgdorferi outer surface protein A (OspA), or immunization with recombinant OspA (Fikrig et al., 1990), protect mice from challenge with in vitro-grown or tick-borne B. burgdorferi. Based largely on the protective efficacy of experimental OspA vaccines in rodent models of Lyme borreliosis, three monovalent OspA-based vaccines are currently in clinical trials. However, recent findings suggest that broad, sustained protection of humans may be difficult to achieve with vaccines based solely on OspA.
Three observations, however, suggest that OspA-based vaccines may prove to have limited efficacy in treating Lyme disease in humans:
a) Modulation of OspA expression by B. burgdorferi may limit the site of action of OspA-specific antibodies to spirochetes residing in the tick midgut as these antibodies are ineffective shortly after infection; PA1 b) Human immune responses to OspA subunit vaccines have not matched those of rodents in level or duration; and PA1 c) OspA is serologically diverse, particularly among European and Asian B. garinii and B. afzelii isolates. Reactivity with panels of OspA monoclonal antibodies (mAbs), and DNA sequence analysis has shown that as many as seven different OspA subgroups can be distinguished (Wilske et al., 1991; 1993). PA1 (1) Serological variation of DBP is less than that of OspA as antibodies reactive with DBP derived from B. burgdorferi sensu stricto are also growth-inhibitory to many strains of B. garinii and B afzelii; PA1 (2) Immunization of mice with either full-length DBP or a recombinant truncated chimeric DBP protects against challenge with a heterologous B. burgdorferi strain; PA1 (3) Antiserum against DBP.sub.297 provides either complete or partial protection against several additional heterologous B. burgdorferi, B. afzelii, and B. garinii strains; PA1 (4) Antibodies (FAb fragments) against DBP block the binding of recombinant E. coli expressing DBP to decorin; and PA1 (5) The dbp gene from B. burgdorferi strain 297 has been used to identify and isolate molecular clones of alleles of this gene present in different phylogenetic groups of Lyme disease spirochetes including B. garinii, B. japonica, B. afzelii, and related Borrelia spp. by utilization of PCR.TM. techniques. These new dbp alleles have been shown to have a high level of sequence identity to the DBP from strain 297. Indication was seen of important serologic differences among the various DBPs suggesting that it may be necessary to formulate a multiple-DBP antigen cocktail to achieve complete vaccine coverage of all clinically relevant B. burgdorferi strains.
Moreover, these variations will no doubt affect the cross-protection to be anticipated with OspA vaccines. Cross-protection was seen by one group using an immunocompetent mouse model (Fikrig et al., 1995), but cross-protection was weak or absent in SCID mouse or hamster models used by other (Schaible et al., 1993; Lovrich et al., 1995). An additional concern is that as many as 10% of B. burgdorferi isolates fail to express OspA in culture (Wilske et al., 1991; 1993).
Another problem with the use of OspA as antigens for stimulation of an immune response in an affected patient is the fact that OspA protein is either poorly immunogenic in humans, or not expressed by B. burgdorferi in vivo until late in infection. Lyme disease patients, mice, hamsters, and dogs infected by tick bite or low-doses of cultured B. burgdorferi fail to mount substantial anti-OspA immune responses for many months following infection although they do mount early responses to other B. burgdorferi antigens (flagellin, OspC, etc.) (Steere, 1989; Barthold and Bockenstedt, 1993). OspA is expressed by B. burgdorferi within ticks (Barbour et al., 1983), but detection of OspA on borrelias in tissue early after infection is difficult. Passive immunization of mice with OspA antibody (Schaible et al., 1990), or immunization with recombinant OspA, after challenge does not eliminate infection and only partially alters disease.
Unfortunately, OspA-immunized mice are not protected from a challenge with host-adapted spirochetes delivered in the form of skin biopsy transplants from infected mice (Barthold et al., 1995). The bacteria appear to express OspA in vivo only at later stages when the infection becomes disseminated. This would be explained by down-regulation of OspA expression by borrelia shortly after initiation of feeding by the tick.
In a recent publication (de Silva et al., 1996) it has been demonstrated that when OspA-specific antibodies were administered to mice before or at the time of attachment of borrelia-infected ticks these mice were protected from spirochetal infection. However, when OspA-specific antibody was administered 48 hr after tick attachment no protection was observed.
Modulation of borrelia antigen expression within feeding ticks has recently been reported for OspC; initially low in resting ticks, OspC levels increase on B. burgdorferi after initiation of tick feeding (Schwan et al., 1995). OspC might appear to be a promising in vivo target, but its high level of antigenic variation complicates its development as a vaccine (Probert and LeFebvre, 1995).
In vitro cultivation of B. burgdorferi suggests that the genes for OspA and OspC are inversely regulated. Preliminary findings of some researchers do suggest that OspA levels similarly decrease after initiation of tick feeding. If these findings are confirmed, OspA antibodies will need to pre-exist at high levels in human or animal hosts prior to the tick bite to be effective against OspA-expressing borrelias in the tick, and may receive little or no boosting upon delivery of the spirochetes into the skin within the milieu of immunosuppressive components of the tick saliva (Urioste et al., 1994).
A recent publication (Telford et al., 1995) describes the efficacy of human Lyme disease vaccine formulations in a mouse model. The authors speculate that "(i)t is likely that titer of circulating antibody to OspA critically determines protection because of the unique mode of action of antispirochetal immunity, wherein antibody or other effectors interfere with the process of transmission within the gut of the infecting tick, before inoculation of the pathogen." Consistent with this hypothesis it has been shown that anti-borrelia serum can protect mice from infection by tick bite if administered within two days after initiation of feeding by borrelia-infected ticks, but not when passively administered at later times (Shih et al., 1995). The antibody levels in response to recombinant OspA subunit vaccinations seen to date in Phase II trials have been moderate, with serum ELISA titers &lt;3,000, and drop off to near baseline levels within five months (Keller et al., 1994). The results in these studies indicate that it will be necessary to include additional antigens to achieve a protective vaccine for Lyme disease.
1.3 Deficiencies in the Prior Art
It is clear that while several approaches to the treatment of bacterial diseases have experienced some success, many problems remain, including antibiotic resistance, variability of antigens between species and species variation through mutation of antigens, as well as the need to protect susceptible groups such as young children, the elderly and other immunocompromised patients. Thus, there exists an immediate need for an effective treatment for B. burgdorferi, and vaccines against the causative agent of Lyme disease.
Although attempts have been made to utilize the Osps as vaccines to confer protection against B. burgdorferi, the results have been disappointing. Because these proteins have demonstrated strain specificity, e.g., variance among isolates and among different passages, and some lack of cross protection between strains, their potential use as vaccines remains very limited.
Because currently known antigens are not sufficient to elicit a protective immune response over a broad spectrum of B. burgdorferi strains, there continues to be an urgent need to develop novel prevention and treatment methods as well as novel antigens able to elicit a broad spectrum immune response and useful diagnostic methods for the prevention, treatment, and diagnosis of Lyme disease.